CPython implementation of intersections

[tomasr8]

This is my stab at a reference implementation of the Intersection type in CPython:
https://github.com/tomasr8/cpython/tree/intersection-type
Contributions/improvements welcome!

Build instructions:

https://devguide.python.org/getting-started/setup-building/
TLDR:

git remote add tomasr8 https://github.com/tomasr8/cpython
git fetch tomasr8
git checkout --track tomasr8/intersection-type
make clean && make -s -j2
./python

TODOs

• Intersection flattening (same as unions)
• fix some copy&paste errors - references to Union and UnionGenericAlias and some docstrings
• isinstance support
• Incorporate gh-105499: Merge typing.Union and types.UnionType python/cpython#105511 when it gets merged

Example

from typing import Intersection

Intersection[int, str]
Intersection[int, float, int]  # == Intersection[int, float] (deduped)
Intersection[int, Intersection[float, str]]  # == Intersection[int, float, str] (flattened)
Intersection[int]  # == int

# overloaded __and__ for types
int & str
type(int & str)

class T: ...
T & int

class A: ...
class B: ...
class C(A, B): ...

a = A()
c = C()

isinstance(a, A & B)
isinstance(c, A & B)

[NeilGirdhar]

Why create the Intersection name at all only to deprecate it immediately? Union only exists because | wasn't available.

[CarliJoy]

Because we can backport Intersection with typing_extensions making it available also to older python versions.
The & will be only available with a newer Python version and nobody wants to way 2 Years for that...

Also some people will be forced to use older version for quite some time and we need backwards compatibility for them.

[NeilGirdhar]

Because we can backport Intersection with typing_extensions making it available also to older python versions.

Right, good point. BTW, you can still use & in annotations by using from __future__ import annotations, but I could see wanting to programmatically generate types.

[NeilGirdhar]

Also, Intersection should probably not be added to Python—just to typing-extensions.

[Gobot1234]

Also, Intersection should probably not be added to Python—just to typing-extensions.

Strongly disagree with this. typing_extensions is meant to be for backporting typing features, not having it in typing would create confusion.

[NeilGirdhar]

Strongly disagree with this. typing_extensions is meant to be for backporting typing features, not having it in typing would create confusion.

Good point. I guess as long as the linters push back, that's enough to discourage unnecessary use of it.

[JelleZijlstra]

Late to the party here, but note that there is precedent for adding something to typing that is immediately sort-of-deprecated: PEP 646 added typing.Unpack so that we could add typing_extensions.Unpack while also adding new syntax that made it unnecessary to ever use typing.Unpack. Analogously we could add a typing.Intersection while simultaneously adding a & operator that means you (almost) never have to use typing.Intersection directly.

[tomasr8]

Just an FYI, I rebased this on the latest CPython main